Channel selection in wireless communications

ABSTRACT

This document discloses a solution for operating a radio communication apparatus that exchanges narrowband control messages with other radio communication apparatuses, each control message comprising a pilot sequence and an identifier identifying a transmitter of the control message. The radio communication apparatus receives a broadband signal through a broadband radio receiver and correlates sub-bands of the received signal so as to detect a narrowband control message within the received broadband signal. Upon detection of the narrowband control message on a sub-band of the received broadband signal, the transmitter of the narrowband control message is determined from the identifier of the narrowband control message. It is also determined from the reception of the narrowband control message on the sub-band that the sub-band is preferred by the transmitter of the narrowband control message, and said sub-band is utilized in data communication with the transmitter of the narrowband control message.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Stage application of InternationalApplication No. PCT/FI2012/050483, filed May 22, 2012, which claimsbenefit to Finnish Application No. FI20115590, filed Jun. 14, 2011,which are incorporated by reference herein in their entirety.

BACKGROUND

Field

The invention relates to the field of radio communications and,particularly, to carrying out a frequency channel selection procedure ina radio communication apparatus.

Description of the Related Art

Modern radio communication systems support operation on a frequencychannel selected from a plurality of frequency channels according to adetermined criterion. Some systems rely on frequency planning where agiven frequency band is assigned to the system, and the system isconfigured to operate exclusively on that frequency band. Such systemsare typically based on using licensed frequency bands. Other systems areconfigured to choose a frequency to be used more adaptively, e.g. on thebasis of scanning for the available (non-occupied) frequencies and,then, transferring control messages related to negotiation of thefrequency band to be used. Such methods increase signalling overhead,particularly in networks comprising numerous network nodes.

SUMMARY

According to an aspect of the present invention, there is provided amethod as specified in claim 1.

According to another aspect of the present invention, there is providedan apparatus as specified in claim 9.

According to yet another aspect of the present invention, there isprovided a computer program product embodied on a computer readabledistribution medium as specified in claim 10.

Embodiments of the invention are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are described below, by way ofexample only, with reference to the accompanying drawings, in which

FIG. 1 illustrates communication between radio communication apparatusesin a radio communication system;

FIG. 2 illustrates an embodiment of a process for carrying out channelselection;

FIG. 3 illustrates an embodiment of a broadband receiver configured toreceive a narrowband single-carrier transmission;

FIG. 4 illustrates a signalling diagram of a channel selection processaccording to an embodiment of the invention;

FIG. 5 illustrates an embodiment of a format of a control message;

FIG. 6 illustrates a signalling diagram of a data transfer negotiationprocess according to an embodiment of the invention;

FIGS. 7 and 8 illustrate formats of a transmission request message and atransmission response message according to some embodiments of theinvention;

FIG. 9 illustrates an example of delay spread of a radio signal;

FIG. 10 illustrates an embodiment of a process utilizing multiple radiopropagation paths;

FIG. 11 illustrates an embodiment of a process for optimization channelutilization; and

FIG. 12 is a block diagram of an apparatus according to an embodiment ofthe invention.

DETAILED DESCRIPTION

The following embodiments are exemplary. Although the specification mayrefer to “an”, “one”, or “some” embodiment(s) in several locations, thisdoes not necessarily mean that each such reference is to the sameembodiment(s), or that the feature only applies to a single embodiment.Single features of different embodiments may also be combined to provideother embodiments. Furthermore, words “comprising” and “including”should be understood as not limiting the described embodiments toconsist of only those features that have been mentioned and suchembodiments may contain also features/structures that have not beenspecifically mentioned.

FIG. 1 illustrates an exemplary wireless telecommunication system towhich embodiments of the invention may be applied. Embodiments of theinvention may be realized in an ad hoc network comprising a plurality ofnetwork nodes 10, 11, 12 that may be realized by radio communicationapparatuses. The ad hoc network may refer to a network that isestablished between the network nodes 10 to 12 without any networkplanning with respect to the infrastructure and/or frequencyutilization. The network nodes may be operationally equivalent to eachother. At least some of the network nodes 10 to 12 are free to move, andthey may also be configured to route data packets that are unrelated totheir own use, e.g. data packets of other network nodes. However, itshould be understood that principles of the invention may be applied toother types of communication systems, e.g. wireless mesh networks,communication systems having a fixed infrastructure such as cellularcommunication systems, and other types of systems. The principles of theinvention may also be applied to point-to-point connections, wherein twonetwork nodes communication directly with each other without using anyother network node to route the data packets.

In the embodiment of FIG. 1, the network nodes 10 to 12 have a very longcommunication range (even thousands of kilometers), and they maycommunicate directly with network nodes on the other side of the Earth.Their transmit powers may vary from a few Watts (e.g. 20 to 50 W) toeven kilo Watts, depending on whether the network node is mobile orfixed and the type of power supply. For example, a network nodeinstalled to a building, a truck, or a ship may utilize high transmitpowers, while a hand-held device may be limited to a few Watts. Thefrequency band utilized by the network nodes 10 to 12 may comprise ahigh frequency (HF) band (3 to 30 MHz), but it should be understood thatother embodiments utilize other frequency bands, e.g. very highfrequencies (VHF) or ultra-high frequencies (UHF). An advantage of HFfrequencies is their long propagation range, and the fact that they maypropagate via several types of communication paths. FIG. 1 illustrates ascenario where a first network node 10 communicates with a secondnetwork node 11 over surface radio waves that propagate close to theground surface. However, a third network node 12 on the other side ofthe Earth may be reached via radio waves that propagate by utilizingionospheric reflections. Some network nodes may be reached by using bothsurface waves and ionospheric reflections, and some embodiments of theinvention are configured to utilize this property.

The network nodes 10 to 12 are configured to support communication on ahigh frequency band from which actual transmission frequencies may beselected according to embodiments described herein. The supportedfrequency band may be continuous or divided into a plurality offrequency bands separated from each other. The division may be based onthe fact that there are other systems occupying some frequencies thatmay have a priority to occupy the frequencies, while the present systemhas to adapt to the frequency occupation of such a primary system. Insome embodiments, the systems occupying the same frequency band haveequal priority to the frequency occupation, and at least the presentsystem may utilize cognitive channel selection procedures describedherein to avoid collisions between the systems. The frequencyutilization is described in greater detail below with reference to FIG.3.

FIG. 2 illustrates a flow diagram of a process for operating a radiocommunication apparatus which may be any one of the network nodes 10 to12. The process may be realized by a computer program executed by acomputer inside the radio communication apparatus. Referring to FIG. 2,the radio communication apparatus is configured to exchange narrowbandcontrol messages with other radio communication apparatuses. Eachcontrol message comprises a pilot sequence and an identifier identifyinga transmitter of the control message. In some embodiments, the controlmessage consists of the pilot sequence and the identifier. The exchangeof the control messages may be carried out repeatedly according topreset rules that may be time-based and/or need-based. With respect tothe operation of the computer as defined by the computer program inblock 202, the computer program may configure the radio communicationapparatus to transmit, receive, and process the narrowband controlmessages, as described in greater detail herein.

Blocks 204 to 210 relate to the reception of a single narrowband controlmessage in the radio communication apparatus. In block 204, theapparatus receives a broadband signal through a broadband radioreceiver. The broadband radio receiver is configured to carry out thereception on a frequency band that is significantly broader than thebandwidth of the narrowband control message. In some embodiments, thebandwidth of the receiver may be more than ten times the bandwidth ofthe narrowband control message, and in other embodiments even hundredsor thousands time the bandwidth of the narrowband control message. Inblock 206, the apparatus carries out a signal detection procedure on thereceived broadband signal so as to detect a narrowband control messagewithin the received broadband signal. The signal detection may becarried out for a plurality of sub-bands of the received broadbandsignal. For example, the received broadband signal may be divided into aplurality of sub-bands having the bandwidth corresponding to the knownbandwidth of the narrowband control message, and the signal detectionprocess may be carried out for each sub-band separately. In practice,the received signal of a given sub-band may be correlated with a pilotsequence stored in a memory of the apparatus. The pilot sequence may bethe same as the pilot sequence added to the narrowband control messagein its transmitter.

Upon detection of the narrowband control message on a sub-band of thereceived broadband signal, the transmitter of the narrowband controlmessage is determined in block 208 from the identifier comprised in thenarrowband control message. The network nodes 10 to 12 may be configuredto transmit the control messages only on the sub-bands that arepreferred sub-bands, e.g. the communication quality of the sub-band ishigh enough. Therefore, a receiver of the control message may determinefrom the sub-band on which the control message was received that thesub-band is preferred by the transmitter of the narrowband controlmessage. Such determination is made in block 210. Therefore, thesub-band may be utilized in data communication with the transmitter ofthe narrowband control message.

As a result of the above-mentioned channel selection procedure, nomanual frequency planning or excessive control signalling related to thenegotiation of the common frequency band(s) to be utilized in thecommunication is necessary. Repeated transmission of the controlmessages also enables fast adaptation to changing radio environment.Typically, one sub-band may have high quality for a given time periodafter which other systems occupy the sub-band, and the quality of thesub-band deteriorates. For example, HF frequencies are susceptible tovarious natural phenomena, e.g. solar activity and other radiationsoriginating from the space, and the other radio systems also contributeto the changing radio environment. Systems with static frequencyplanning cannot adapt to such changes and, therefore, their performancedegrades. Furthermore, the radio environment may be completely differentfor two network nodes far away from each other. This raises therequirements for the fast adaptation, as the probabilities that at leastone of two network nodes experiences degradation of current sub-bands isincreased. The radio communication apparatus may, upon detection of poorperformance in the currently used sub-band(s), scan for better sub-bandsand transmit the control messages on new sub-bands detected to havebetter quality. Upon reception of the control messages on new sub-bands,a receiver apparatus may update the preferred channel list accordingly.The channel selection process comprising the exchange of the controlmessage(s) and the processing of the received control message(s) in thereceiver may take even less than 200 ms which enables fast adaptation tothe changing radio environment and necessarily no negotiation other thanthe unidirectional transmission of the control message. However,additional signalling may be used in connection with data transmission,as will be described below.

Let us consider the frequency utilization and the operation of the radiocommunication apparatus in greater detail with reference to FIG. 3. FIG.3 illustrates that the operational band of the whole system is dividedinto a plurality of frequency blocks, each frequency block having anexemplary 192 kHz bandwidth. The radio communication apparatus is tunedto receive 192 kHz signals of each frequency block. The radiocommunication apparatus may comprise a plurality of radio receivers 30,31, 32, wherein each radio receiver 30 to 32 is tuned to receive radiosignals on at least one frequency block. In some embodiments where thenumber of frequency blocks supported by the system is higher than thenumber of radio receivers 30 to 32, at least some of the radio receivers30 to 32 are tuned to receive a plurality of frequency blocks. The radioreceivers 30 to 32 may then carry out frequency-hopping between saidplurality of frequency blocks. The bandwidth of the actual transmissionsis 3 kHz in this embodiment. Each 192 kHz frequency block is divided to3 kHz (1 kHz or another bandwidth in other embodiments) sub-bands. Insome embodiments, the number of sub-bands in the frequency blocks is thebandwidth of the frequency block divided by the bandwidth of thesub-band, e.g. 192 kHz/3 kHz=64. In such embodiments, the separationbetween centre frequencies of adjacent sub-bands is equal to thebandwidth of the sub-bands, e.g. 3 kHz. However, in more efficientembodiments, the separation between centre frequencies of adjacentsub-bands is lower than the bandwidth of the sub-bands. This effectivelymeans that the sub-bands overlap in the frequency domain, but sufficientfrequency separation may still be achieved so that adjacent channelinterference may be mitigated in the receiver. For example, the centrefrequency separation may be 1 kHz or even 500 Hz, while the bandwidth ofthe sub-band is several kHz. In other embodiments, a guard band isprovided between adjacent sub-bands. The transmitter may be configuredto select one or more sub-bands per frequency block to carry out thetransmission. If a frequency block does not contain an unoccupiedsub-band, the frequency block may be bypassed in the selection of thesub-band. As a consequence, the number of sub-bands used in thetransmission may be equal to the number of frequency blocks supported bythe system. However, as mentioned above, the number of sub-bands used inthe transmission may be other than the number of frequency blocks, whenzero to more than one sub-band may be selected per frequency block. Thesub-bands used in the transmission are typically non-consecutive exceptfor in special cases, e.g. when the highest sub-band of a firstfrequency block and the lowest sub-band of a neighbouring frequencyblock on a higher frequency are selected for the transmission.

As the transmitter may select the sub-bands on which to transmit thecontrol messages, each broadband receiver 30 to 32 do not necessarilyknow on which one of the 64 (or 192) sub-bands of the frequency blockthe transmission is located. As a consequence, each receiver branch maycomprise a matched filter 33, 34, 35 matched to the waveform of thepilot sequence and configured to scan for the (3 kHz) sub-bands of thereceived broadband (192 kHz) signal and to detect the pilot sequenceknown to be comprised in the control message. Each network node 10 to 12may utilize the same pilot sequence in order to reduce the complexity ofthe matched filter structure 33 to 35. The structure of the pilotsequence is described in greater detail below with reference to FIG. 5.

In an embodiment, the receiver utilizes time-domain correlation, whereineach radio receiver 30 to 32 divides the received 192 kHz broadbandsignal into 3 kHz sub-bands by using a bandpass filter structure whichmay be realized by a filter bank dividing the received signal into aplurality of (3 kHz) sub-band signals, for example. Then, the narrowband3 kHz signals are applied to a corresponding matched filter 33, 34, or35, and the matched filter carries out a correlation with each 3 kHzsignal so as to detect a correlation peak that would indicate thepresence of the pilot sequence in the received signal. In order todetect the correlation peak, the matched filters 33 to 35 may employ apeak detector comparing the result of the matched filtering with athreshold value. A result exceeding the threshold is considered as adetection of the pilot sequence in the received signal.

In another embodiment, the time-domain correlation is replaced by a(fast) Fourier transform of the received signal and a multiplicationbetween the transformed received signal and a frequency-domainrepresentation of the pilot sequence. This type of filtering proceduremay employ the known overlap-and-add method or overlap-and-save method.

In another embodiment, the receiver utilizes an OFDM (OrthogonalFrequency Division Multiplexing) or, in general, a multicarrier receiverstructure designed for receiving multicarrier signals, that is signalshaving symbols on a plurality of parallel orthogonal sub-carriers. Asthe symbols are separated in frequency, the OFDM receiver is typicallyconfigured to process the received signals in a frequency domain. TheOFDM receiver may be tuned to receive a frequency block (192 kHz), andit may be configured to consider each (3 kHz) sub-band as a“sub-carrier”. As a consequence, a single-carrier control message isreceived with a multi-carrier receiver. As the OFDM receiver processesthe received signal in the frequency domain, the radio receiver 30 to 32may comprise a Fourier transform circuitry configured to convert thereceived signal into a frequency domain representation. A time windowfor the Fourier transform may be selected to be the duration of thepilot sequence of the narrowband control message. Thereafter, thematched filters 33 to 35 matched to the waveform of a frequency-domainrepresentation of the pilot sequence process each sub-band. In thefrequency domain, the matched filtering procedure comprises a simplemultiplication between the received signal and the pilot sequence,thereby providing computationally less complex correlation than with aconvolution used in the time-domain correlation.

As known in the art, the matched filters 33 to 35 may be replaced by acorrelator structure.

Upon detection of the pilot sequence in one of the sub-bands of thereceived signal, the sub-band signal is applied to a control messageprocessor 36 that may be configured to process the sub-band signal. Theprocessing may comprise applying receiver signal processing algorithms,e.g. equalization, to the sub-band signal. The pilot sequence containedin the received sub-band signal may be used as a training sequence forthe equalization (a channel response may be estimated from the pilotsequence) and for other signal processing algorithms. Then, the controlmessage processor 36 may extract a payload portion of the controlmessage contained in the sub-band signal and recover an identifiercontained in the payload portion. Upon deriving the transmitter of thecontrol message from the identifier, the control message processor 36may store in the memory 37 the corresponding sub-band as a preferredchannel for that transmitter. Thereafter, that sub-band may be used incommunication with the transmitter.

Let us now consider the channel selection process on a higher level withreference to FIG. 4. FIG. 4 illustrates a signalling diagramillustrating how a transmitter and a receiver, which both may be thenetwork nodes 10 to 12 of FIG. 1, determine the channel(s) over which tocommunicate with one another. Let us note that the terms transmitter andreceiver are only related to the transmitter of the control message andthe receiver of the control message, and the roles may be reversed inother contexts, e.g. in the data transmission or transmission of acontrol message to the other direction. Referring to FIG. 4, thetransmitter first carries out in S1 a scanning process on some or allchannels supported for communication. The transmitter determines in S1the channel(s) that provide the highest channel quality. The transmittermay be configured to select one channel per frequency block, wherein theselected channel may have the highest channel quality within thefrequency block. Quality estimates may be based on estimating receivedsignal strength on each channel, signal-to-noise ratio (SNR) orsignal-to-interference-plus-noise ratio (SINR) on each channel, or onany other channel quality metric.

In S2, the receiver scans the frequency blocks and correspondingsub-bands with the broadband radio receiver for presence of narrowbandcontrol messages, as described above. S1 and S2 are mutually independentprocesses, and their respective timings may vary, e.g. S2 may be carriedout before or at the same time as S1. Upon selecting the channel(s) orsub-bands of frequency blocks in S1, the transmitter transmits in S3 oneor more control message(s) on the selected channel(s). The controlmessages may be broadcast messages that are not addressed to anyspecific receiver. As a consequence, any network node capable ofreceiving the control message may process the control message. Uponreception of the control message(s) on the respective channel(s) in S3,the receiver detects the control message(s) and processes them in S4 soas to derive the identifier of the transmitter from the message(s) andto carry out association between the channel(s) and the transmitter. Asa consequence, the receiver derives a list of channels preferred by thetransmitter. Then, the receiver determines the channels it prefers inS5. This may be obtained through a process similar to the one carriedout in S1. The receiver may then make a comparison between the channelspreferred by the transmitter and the channels preferred by the receiver.The commonly preferred channels may then be used in data transferbetween the transmitter and the receiver in S6. Sometimes, the channelspreferred by transmitter and the receiver overlap only partially (or donot overlap at all), and in such cases the receiver may determine S6those channels that provide the best channel quality for the receiver.The channel selection in connection with the data transfer is describedin greater detail below.

The channel selection of FIG. 4 may be carried out repeatedly byexchanging the control messages between the network nodes 10 to 12. Thetransmission of new control messages may be triggered on a time basisand/or on a need basis, as mentioned above. For example, a network nodemay be configured to carry out the channel selection processperiodically. In another example, the network node may be configured tocarry out the channel selection upon degradation of at least some of thecurrently selected channels. A trigger may be that there is less than adetermined number of channels with sufficient quality, and thesufficient quality may be determined by a given metric (e.g. SINR and/orbit/packet error rate) and a threshold. Other criteria for triggeringthe selection of new channels may also be used.

FIG. 5 illustrates an embodiment of a format of the narrowband controlmessage. As mentioned above, the control message may comprise the pilotheader and the payload comprising the identifier of the transmitter ofthe control message. The pilot header may comprise a plurality ofconcatenated pilot sequences to provide a long pilot sequence thatoccupies more than half of the length of the control message. In anembodiment, the pilot header comprises four concatenated pilotsequences, wherein each pilot sequence has the length of 32 chips. Thesequences may be direct sequence (DS) codes used in spread spectrumcommunications. Examples of the sequences that may be used as the DSpilot sequence include m-sequences and Gold sequences. However, in thiscase the DS sequences are not used to increase the symbol rate of thecontrol message, contrary to how they are used in the spread spectrumcommunications. In fact, the symbol rate of the pilot sequence may bethe same as the symbol rate of the payload portion. In the example offour 32-chip sequences and a 16-bit identifier and a chip lengthequaling the bit length, the total length of the control message is 144bits. In the example of 3 kHz narrowband control message, the symbolrate may be 3 ksps (kilosymbols per second). The long pilot sequencemaximizes the probability of receiving the control message in thereceiver. Remember that each radio receiver may scan for a plurality offrequency blocks and, possibly, is not able to monitor each channelcontinuously. Using a long pilot sequence then improves the probabilityof detecting the control message. The matched filter of the receiver maybe matched with a single pilot sequence, and not necessarily with thewhole concatenated pilot part of the control message. Long pilotsequence also enable determining whether the received signal is asurface wave or an ionospheric reflection, better channel estimation,etc.

The payload may also comprise an encrypted time stamp to suppressrepetition interference. A control message having an expired time stampmay be considered as interference and, thus, be discarded.

In some embodiments, the pilot header may be unique for each networknode and, thus, the pilot header functions as the identifier of thenetwork node. Thus, the payload may even be omitted, and the controlmessage may consist of the unique pilot header. In other embodiments,the pilot header may be common to at least some of the network nodes,and the payload may comprise a unique (DS) sequence which may functionas the identifier.

Let us now consider the data transmission in the network according to anembodiment of the invention with reference to FIG. 6. Let us assume thatthe channel selection through the transmission of the narrowband controlmessages has been conducted in the above-described manner. The channelselection is carried out in S11, and it may be carried out in both thetransmitter and the receiver. In S12, the transmitter determines tocarry out a data transmission. Parameters of the data transmission maybe negotiated through a negotiation phase in which the transmittertransmits a transmission request message (e.g. a request-to-send, RTS)to the receiver, and the receiver responds with a transmission responsemessage (e.g. a clear-to-send, CTS). In S12, the transmitter transmitsthe RTS message to the receiver. The RTS message may be transmitted on aplurality of channels, e.g. on a sub-band of every frequency block forwhich the transmitter has selected a sub-band.

FIG. 7 illustrates an embodiment of the RTS message. The RTS message maycomprise the same pilot sequence as a header as the control message ofFIG. 5. As the payload portion, the RTS message may comprise theidentifier of the transmitter and the identifier of the recipient of theRTS message, which both may be unique for each network node 10 to 12.The RTS message may also comprise an information element used to specifyhow much data the transmitter needs to transmit. This informationelement may be used to define a quality-of-service (QoS) classificationof the data being transmitted. The QoS classification may specifyreal-time requirements for the data, and typical QoS classifications mayinclude conversational and streaming as real-time classes andinteractive and background as non-real time classes. Other QoS classesare equally applicable. The RTS message may further comprise a fieldspecifying at least one channel to be used as a feedback channel for atleast the CTS message but, optionally, also for the data transmission.This field may be used by the transmitter to specify at least one (butin some embodiments a plurality, e.g. four) feedback channel on whichthe CTS message is to be transmitted. Furthermore, the feedbackchannel(s) may be used to convey positive/negative acknowledgmentmessages (ACK/NACK) indicating successful/erroneous data reception,respectively.

Upon reception of the RTS message in S12, the receiver detects the RTSmessage in S13 on the basis of the matched filtering the pilot sequence,as described above. Furthermore, the receiver may detect from thestructure or from a specific identifier contained in the message thatthe message is the RTS message and not the conventional control messageof FIG. 5. Upon determining that the message is the RTS message, thereceiver extracts the payload portion of the RTS message and processesthe transmission request. The extraction may again comprise equalizationbased on using the pilot header as the training sequence, and alsosynchronization with symbol timing of the RTS message may be carried outon the basis of the pilot header, as may be done with the controlmessage of FIG. 5.

In S13, the receiver detects the QoS classification of the request (oranother indicator specifying the amount of transmission resourcesneeded), determines the number of sub-bands needed to comply with therequest, and selects the sub-bands. The selection of the sub-bands maybe based on selecting the necessary number of sub-bands that aredetermined to provide the highest channel quality for the transmitter orfor the transmitter and the receiver. Again, one sub-band per frequencyblock may be selected for the data transfer, but in other embodimentsmultiple sub-bands per frequency block may be selected. Additionally,the receiver may determine a modulation and coding scheme that providesa data rate that complies with the QoS class specified in the RTSmessage. The receiver may determine the modulation and coding scheme(and other transmission parameters) also (or alternatively) on the basisof the channel state of the selected sub-band(s). In order to reduce thecomplexity of the receiver, the receiver may be configured to support alimited number of modulation and coding schemes. In some embodiments,the modulation and coding scheme may be a semi-static parameterdetermined periodically, and the same modulation and coding scheme maybe applied to consecutive data transmissions regardless of the changingchannel state. However, better spectral efficiency is achieved by usinga dynamically selected modulation and coding scheme. The selection ofthe number of sub-bands and the modulation and the coding schemes may bea compromise, wherein a lower number of selected sub-bands may becompensated by a higher order modulation and coding scheme (whichprovides higher data rates). Similarly, if a high number of sub-bands isavailable, even a lower order modulation and coding scheme providing alower data rate but improved reliability may be used.

In S14, the receiver prepares the CTS message for transmission to thetransmitter. FIG. 8 illustrates an embodiment of the format of the CTSmessage. The CTS message may comprise the above-mentioned pilot header,but the number of concatenated pilot sequences contained in the pilotheader may be different than in the control message and the RTS message.As the transmitter has already specified the sub-band(s) for the CTSmessage, it is configured to monitor for those sub-bands for the CTSmessage. Therefore, a shorter pilot header may be used in the CTSmessage. The payload part of the CTS message may comprise the identifierof the receiver (the transmitter of the CTS message), the channelallocation for the data transmission comprising the selected sub-bands,and the selected modulation and coding scheme. The channels may beidentified by using channel indexes, wherein each sub-band has a uniquechannel index. Similarly, the modulation and coding schemes may beindexed, and the appropriate index may be specified in the CTS message.In S14, the receiver transmits the CTS message to the transmitter on thechannel(s) specified in the RTS message. It should be noted that thechannel allocation specified in the CTS message may specify at leastsome different channels than those specified in the RTS message for thetransmission of the CTS message and the ACK/NAKs. The transmitterreceives the CTS message in S14. The transmitter uses the pilot headerfor timing synchronization and/or for the equalization of the CTSmessage, and extracts the payload part of the CTS message. Then, thetransmitter configures its transmitter parts for transmission with theparameters specified in the CTS message.

In S15, the transmitter carries out the data transmission on thesub-band(s) allocated in the CTS message by using the modulation andcoding scheme specified in the CTS message. The receiver is configuredto monitor for those sub-bands. Upon reception of the data transfer onthose channels, the receiver processes the received data by carrying outdata detection and decoding algorithms. Upon successful reception of thedata, the receiver is configured to transmit an ACK message on thesub-band(s) specified in the RTS message. However, upon erroneousreception of the data, the receiver is configured to transmit a NAKmessage on the sub-band(s) specified in the RTS message. In someembodiments, the receiver responds only to the correct reception (ACK)or to the erroneous reception (NAK) of the data. For example, when thereceiver acknowledges only the correct receptions by transmitting ACK,the transmitter detects erroneous reception upon detection of no ACKmessage for a given data packet. Any hybrid automatic repeat request(HARQ) procedures are also possible, wherein upon detecting erroneousreception of a data packet, a retransmission comprises either the samedata packet (chase combining) or additional information (e.g. paritybits) that help the decoding in the receiver. The latter embodiment isknown as incremental redundancy HARQ.

In this manner, the data transfer continues between the network nodes 10to 12. The other network nodes not part of the data transfer between thetransmitter and the receiver may also be configured to monitor for atleast the RTS messages. After all, all the network nodes monitor fortransmissions and receive radio signals, process them to some degreeafter they determine whether or not the message concerns them. Forexample, a network node may extract a message to some degree before itis able to determine whether the message is the control message of FIG.5 or the RTS message of FIG. 7. In an embodiment, the network node mayutilize this feature to such degree that if the received message is thecontrol message of FIG. 5, the network node may update the channel listused with the transmitter of the control message. However, if thereceived message is the RTS message not addressed to the network node,the network node may extract the identifier of the transmitter of theRTS message and determine that the transmitter of the RTS message isreserved for a determined period of time. If the RTS message containsinformation that enables the determination of the time period thetransmitter is reserved, the network node may use that information toderive the reservation period for the transmitter. In other embodiments,the network node may set a default reservation period. Similarly, thenetwork node may derive the identifier of the receiver of the RTSmessage, and set a corresponding reservation period for the receiver. Asa consequence, the RTS message may be used as an indicator that thetransmitter and the receiver are reserved and no data should betransmitted to them during the reservation period. In some embodiments,the network node is configured to disregard the CTS messages and, as aconsequence, the channel(s) or sub-bands specified in at least the CTSmessage. Accordingly, the network node may carry outtransmission/reception on those sub-bands. In other embodiments, thenetwork node is configured to extract the CTS messages as well, and toprevent transmission on the sub-bands allocated to the data transfer ortransfer of ACK/NAKs that are specified in the RTS and the CTS messages.As a consequence, the RTS and CTS messages may be used to carry outchannel reservation and protection of the data transfer.

The physical layer channel selection principles of the above-describedembodiments may also be utilized in equipping the network nodes 10 to 12with link layer and/or network layer intelligence. As each channel maybe utilized by a plurality of network nodes, each network node may beequipped with Medium Access Control (MAC) logic realizing, for example,a carrier sense multiple access (CSMA) procedure in which the networknode senses the sub-bands it intends to in the transmission prior tocarrying out the transmission on those sub-bands. If the sub-band isdetected to be free, the network node proceeds to transmission. On theother hand, if the sub-band is detected to contain interference (e.g.another user/system), the network node may tune to another sub-band andcarry out the CSMA on that channel. The channels sensed in the CSMAprocess may be the channels allocated to be preferred by the networknodes carrying out the data transfer over a radio link. The network nodemay also employ collision detection and/or collision avoidanceprocedures to avoid collisions. This may be applied to the transmissionof the control message, the RTS message, the CTS message and/or thedata. The channel selection procedure in the RTS/CTS handshake isanother example of the MAC procedures implemented in the network. Withrespect to the network layer, as each network node 10 to 12 stores alist of other network nodes with which it is able to communicate, thenetwork nodes may be configured to exchange routing messages. A routingmessage may comprise a list of network nodes a given network node 10 to12 is able to reach, either directly or indirectly. This enables theother nodes to construct a routing table comprising a list of nodes thatmay be reached through a given neighbour node. The routing tables may beused to determine routes in the ad hoc network, e.g. by determiningthrough which node a given destination node may be reached. Thus, therouting tables may be used in transmitting and forwarding the datapackets. The routing tables may be seen as higher layer signalling, andthe routing tables may be transmitted as data in the physical layer. Asa consequence, the transmission of the routing table may be carried outthrough the RTS/CTS handshake procedure.

In the embodiments utilizing the HF frequencies, the presence of theionospheric reflection as a radio path is available. The receiver of thecontrol message(s) is able to determine the presence or absence of theionospheric reflection from the received control message, if the pilotsequence is sufficiently long. Therefore, the length of a pilot sequencecomprised at least in the control message and in the RTS message may beselected to be longer in time than the longest expected delay in thesignal propagation, e.g. the length of the pilot sequence may be up to 6ms. As mentioned above, the pilot header may comprise a plurality ofsuch concatenated pilot sequences. FIG. 9 illustrates correlation peaksrepresenting signal components of the transmitted control message thatarrive at the receiver at different time instants, wherein each peakrepresents a signal component traveled through a different signal pathin the radio channel. Surface waves that propagate approximately througha direct path from the transmitter to the receiver typically arrivebefore the ionospheric reflections that propagate a longer distance tothe ionosphere and back to the ground surface. This shows as differentsignal groups in the delay spread illustrated in FIG. 9. A signal groupconsisting of the surface waves is typically separated from the signalgroup consisting of the signal components reflected from the ionosphere.Any state of the art signal grouping algorithm may be applied to thematched filter outputs determine whether there is one signal groupindicating the presence of only the surface waves or the ionosphericreflection or two signal groups indicating the presence of both thesurface waves and the ionospheric reflection. This information may beused in the data transfer according to some embodiments, as illustratedin FIG. 10. Other embodiments for detecting the ionospheric reflectionsmay be utilized as well.

Referring to FIG. 10, the matched filter outputs with different symboltimings are analysed and, thus, the correlation peaks for each signalcomponent is derived, e.g. as illustrated in FIG. 9. These correlationpeaks are then analysed in block 102 so as to determine whether there isone signal group or two signal groups. On the basis of this analysis,different communication procedures or parameters are applied in block104. For example, when the analysis in block 102 indicates that bothionospheric reflection waves and surface waves are available for a givennetwork node, the communication parameters may be optimized for thetransmission of the surface waves or the ionospheric reflection waves.The radio environments are different when transmitting the surface wavesand the ionospheric reflection waves. For example, when the ionosphericreflection waves are available between the two network nodes 10 to 12,the radio environments of the network nodes 10 to 12 are typically verydifferent. This results from the fact that ionospheric reflections aretypically present between two network nodes located far from each other(hundreds or even thousands of kilometers). For example, a first networknode may have completely different set of preferred sub-bands than asecond network node because of different interference scenarios. Whenthe ionospheric reflection waves are available, the sub-bands of thedata transmission may be selected exclusively on the basis of thereceiver's preferred sub-bands regardless whether or not those sub-bandsare preferred by the transmitter. The radio environment of thetransmitter is assumed not to extend to the receiver and, therefore,sub-bands not preferred by the transmitter may be selected. On the otherhand, if the surface waves are preferred over the ionospheric reflectionwaves in order not to cause unanticipated interference to other networknodes, sub-bands on higher frequencies may be selected for the datatransfer, because the ionospheric reflections diminish on the higherfrequencies. In yet another embodiment, the network node may determinewhether or not to carry out the CSMA procedure (or another channelsensing procedure) prior to the transmission. For example, if it isknown that a destination node may be reached through the ionosphericreflection wave, a source node may select those sub-bands that arepreferred by the destination node and transmit on those sub-bandswithout the channel sensing. The channel sensing may be disregardedbecause of the assumed different radio environments between the sourceand destination node. Therefore, even though the channel sensing showsthat the sub-bands are occupied in the environment of the source node,they may be free at the destination node (because the other interferencemay be assumed not to reach the destination node, which means that thescanning is not necessary with respect to the operation of the linkbetween the source and the destination node. This assumption may bebased on the assumption that the present system utilizes higher transmitpowers than the other systems on the same band. In another embodiment,geocasting may be achieved by appropriate selection of a sub-band. Forexample, if a receiver in a given location is able to utilize a sub-bandthrough the ionospheric reflections, a transmitter may carry outgeocasting by selecting that sub-band and transmitting geocastingmessages (data or control messages) to other receivers in the same areaon that sub-band (and any other corresponding sub-band). Such geocastingreceivers even need not be part of any multicast group. Utilizing thegeocasting may require at least rough knowledge of the locations of thenetwork nodes, and this may be achieved through any state-of-the-artpositioning system (e.g. GPS and exchange of location informationbetween the network nodes). According to another point of view, reversegeocasting may be achieved by the appropriate selection of a sub-band soas to prevent reception of messages in a given geographical area e.g. insome military applications. For example, if a network node in a givenlocation indicates not being able to utilize a sub-band and if anundesired receiver is known to be in or near the same location, atransmitter may select that sub-band and transmit on that sub-band amessage that is not desired to be captured by the undesired receiver.

A network node 10 to 12 may carry out the exchange of the controlmessages with several other network nodes. As a consequence, the networknode acquires a database comprising a list of sub-bands each networknode prefers. Some embodiments of the invention utilize this list ofpreferred sub-bands to reduce transmission overhead. FIG. 11 illustratesa flow diagram of such a procedure. Let us assume that the database ofthe sub-bands for each reachable network node has already beenconstructed. Referring to FIG. 11, a group transmission is initiated inblock 112. The group transmission may refer to a multicast or abroadcast transmission. The basic difference is that the multicasttransmission is a group transmission addressed to a plurality ofdestination nodes, while a broadcast is a group transmission addressedto no specific node, and every node capable of receiving the broadcastmay extract it. The group transmission may be related to thetransmission of payload data (e.g. application data), or it may relateto transmission of higher layer signalling information, e.g. the routingtables.

In block 114, a channel selection procedure is carried out so as tominimize the number of transmissions. It may not be feasible to carryout the group transmission on all sub-bands, because the number ofsub-bands may be even thousands, and most of them may not even becurrently usable by the reachable network nodes because of interference,for example. Therefore, it is not feasible to transmit on sub-bands thatare not available to any other node. Furthermore, the channel selectionprocedure in block 114 determines the sub-bands that are preferred bythe highest number of intended recipients. This procedure is aminimization procedure in which the number of sub-bands used to carryout the group transmission is minimized. In other words, in block 114the minimum number of sub-bands with which all intended recipients maybe reached are selected. Let us assume that a single sub-band may beused to convey the information of the group transmission. Then, thesub-band that is preferred by the highest number of intended recipientsis selected. Thereafter, the recipients still remaining are determined,and the next sub-band preferred by the highest number of remainingrecipients is selected, and so on. When a plurality of sub-bands areused to carry out the group transmission, a more complex procedure maybe used, wherein said plurality of sub-bands are selected per recipient.However, the operation of the procedure in such cases is straightforward. The channel selection may be carried out in the above-describedmanner, and when the necessary number of sub-bands has been selected fora given recipient, that recipient is excluded from the list of remainingrecipients. After the sub-bands have been selected for the grouptransmission, and the group transmission is carried out on the selectedsub-bands in block 116.

FIG. 12 illustrates an embodiment of an apparatus comprising means forcarrying out the functionalities of the network node according to anyone of the above-described embodiments. The apparatus may be a radiocommunication apparatus implemented as a portable device, e.g. acomputer (PC), a laptop, a tabloid computer, a portable radio phone, amobile radio platform (installed to a vehicle such as a truck or aship), or any other apparatus provided with radio communicationcapability. In some embodiments, the apparatus is the vehicle equippedwith the radio communication capability. In other embodiments, theapparatus is a fixed station, e.g. a base station. In furtherembodiments, the apparatus is comprised in any one of theabove-mentioned apparatuses, e.g. the apparatus may comprise acircuitry, e.g. a chip, a processor, a micro controller, or acombination of such circuitries in the apparatus.

The apparatus may comprise a communication controller circuitry 60configured to control the communications in the communication apparatus.The communication controller circuitry 60 may comprise a control part 64handling control signalling communication with respect to establishment,operation, and termination of the radio connections. The control part 64may also carry out any other control functionalities related to theoperation of the radio links, e.g. transmission, reception, andextraction of the control messages and the RTS/CTS messages. Thecommunication controller circuitry 60 may further comprise a data part66 that handles transmission and reception of payload data over theradio links. The communication controller circuitry 60 may furthercomprise a medium access controller circuitry 62 configured to carry outthe channel selection procedures described above. For example, themedium access controller circuitry 62 may determine the sub-bands to beused in the data transfer on the basis of sub-band preferences. Themedium access controller circuitry 62 may also determine the contentsfor the RTS/CTS messages, e.g. the channel selection, the QoSclassification (may be received from higher layers), the modulation andcoding scheme, etc. The communication controller circuitry 60 mayfurther comprise a routing controller circuitry 63 configured to carryout network layer procedures. The routing controller may control thedata part 66 with respect to the transmission of the data. The routingcontroller circuitry 63 may construct the above-mentioned routing tableson the basis of routing messages received from the neighbour nodesand/or other messages the apparatus detects (e.g. RTS/CTS messages). Asa consequence, the routing controller circuitry 63 is configured tocontrol the data part 66 to transmit a given data packet to anappropriate neighbour node.

The circuitries 62 to 66 of the communication controller circuitry 60may be carried out by the one or more physical circuitries orprocessors. In practice, the different circuitries may be realized bydifferent computer program modules. Depending on the specifications andthe design of the apparatus, the apparatus may comprise some of thecircuitries 60 to 66 or all of them.

The apparatus may further comprise the memory 68 that stores computerprograms (software) configuring the apparatus to perform theabove-described functionalities of the network node. The memory 68 mayalso store communication parameters and other information needed for theradio communications. For example, the memory may store the routingtables and/or the list of preferred frequencies for each neighbour node.The memory 68 may serve as the buffer for data packets to betransmitted. The apparatus may further comprise radio interfacecomponents 70 providing the apparatus with radio communicationcapabilities with other network nodes. The radio interface components 70may comprise standard well-known components such as amplifier, filter,frequency-converter, analog-to-digital (A/D) and digital-to-analog (D/A)converters, (de)modulator, and encoder/decoder circuitries and one ormore antennas. In particular, the radio interface components 70 mayrealize the above-mentioned radio receivers 30 to 32, while the matchedfilter and other signal processing may be carried out by any one of theradio interface components 70, the control part 64, and the data part66, according to the design of the apparatus. The apparatus may furthercomprise a user interface enabling interaction with the user. The userinterface may comprise a display, a keypad or a keyboard, a loudspeaker,a smartcard and/or fingerprint reader, etc.

As used in this application, the term ‘circuitry’ refers to all of thefollowing: (a) hardware-only circuit implementations, such asimplementations in only analog and/or digital circuitry, and (b) tocombinations of circuits and software (and/or firmware), such as (asapplicable): (i) a combination of processor(s) or (ii) portions ofprocessor(s)/software including digital signal processor(s), software,and memory(ies) that work together to cause an apparatus to performvarious functions, and (c) to circuits, such as a microprocessor(s) or aportion of a microprocessor(s), that require software or firmware foroperation, even if the software or firmware is not physically present.

This definition of ‘circuitry’ applies to all uses of this term in thisapplication. As a further example, as used in this application, the term“circuitry” would also cover an implementation of merely a processor (ormultiple processors) or portion of a processor and its (or their)accompanying software and/or firmware. The term “circuitry” would alsocover, for example and if applicable to the particular element, abaseband integrated circuit or applications processor integrated circuitfor a mobile phone or a similar integrated circuit in server, a cellularnetwork device, or other network device.

In an embodiment, the apparatus carrying out the embodiments of theinvention in the communication apparatus comprises at least oneprocessor and at least one memory including a computer program code,wherein the at least one memory and the computer program code areconfigured, with the at least one processor, to cause the apparatus tocarry out the steps of any one of the processes of FIGS. 2 to 11.Accordingly, the at least one processor, the memory, and the computerprogram code form processing means for carrying out embodiments of thepresent invention in the communication apparatus.

In an embodiment, the at least one memory and the computer program codeare configured, with the at least one processor, to cause the apparatusto cause a radio communication apparatus to exchange narrowband controlmessages with other radio communication apparatuses, each controlmessage comprising a pilot sequence and an identifier identifying atransmitter of the control message; to acquire a broadband signalthrough a broadband radio receiver and correlating sub-bands of thereceived signal so as to detect a narrowband control message within thereceived broadband signal; upon detection of the narrowband controlmessage on a sub-band of the received broadband signal, to determine thetransmitter of the narrowband control message from the identifier of thenarrowband control message; and to determine from the reception of thenarrowband control message on the sub-band that the sub-band ispreferred by the transmitter of the narrowband control message, and tocause the radio communication apparatus to utilize said sub-band in datacommunication with the transmitter of the narrowband control message.

The term “narrowband” may be defined with respect to the “broadband”such that the bandwidth of the narrowband control message is lower thanthe bandwidth of the broadband radio receiver. According to anotherpoint of view, the narrowband may be defined with respect to itstransmission frequency, e.g. the bandwidth of the narrowband controlmessage is 10% or less than the centre frequency carrying the controlmessage. On the other hand, the bandwidth of the broadband radioreceiver is higher than 10% of the centre frequency of the controlmessage.

In an embodiment, the at least one memory and the computer program codeare configured, with the at least one processor, to cause the apparatusto determine, as a result of said correlation, whether the narrowbandcontrol message was received as a surface radio wave or as anionospheric reflection wave; and to cause the radio communicationapparatus to utilize a different communication procedure depending onwhether a communication link with the transmitter is the surface radiowave or the ionospheric reflection wave.

In an embodiment, the at least one memory and the computer program codeare configured, with the at least one processor, to cause the apparatusto divide a frequency band into a plurality of frequency blocks, eachfrequency block comprising a plurality of sub-bands; to scan saidfrequency blocks and sub-bands and estimate communication quality forsaid sub-bands; to select a sub-band per each frequency block; and tocause the radio communication apparatus to transmit said control messageon each selected sub-band.

In an embodiment, the at least one memory and the computer program codeare configured, with the at least one processor, to cause the radiocommunication apparatus to repeatedly transmit the control messages onselected sub-bands, thus indicating to other radio communicationapparatus that the selected sub-bands are preferred for the radiocommunication.

In an embodiment, the at least one memory and the computer program codeare configured, with the at least one processor, to cause the apparatusto negotiate about data transmission by exchanging a transmissionrequest message and a transmission response message responsive to thetransmission request message, wherein the transmission request messageis transmitted by a transmitter apparatus and comprises parameters ofthe data transmission requested by the transmitter, and wherein thetransmission response message comprises at least a channel allocationfor the data transmission.

In an embodiment, the at least one memory and the computer program codeare configured, with the at least one processor, to cause the apparatusto cause the radio communication apparatus to carry out channel sensingprior to any transmission, except for in special cases described above.

In an embodiment, the at least one memory and the computer program codeare configured, with the at least one processor, to cause the apparatusto cause the radio communication apparatus to carry out a grouptransmission, and to select sub-bands for the group transmission througha minimization procedure where sub-bands are selected in an order ofhighest common preference by intended recipients until channels areselected for the intended recipients. Thereafter, the apparatus causesthe radio communication apparatus to carry out the group transmission onthe selected sub-bands.

The processes or methods described in FIGS. 2 to 11 may also be carriedout in the form of a computer process defined by a computer program. Thecomputer program may be in source code form, object code form, or insome intermediate form, and it may be stored in some sort of carrier,which may be any entity or device capable of carrying the program. Suchcarriers include a record medium, computer memory, read-only memory,electrical carrier signal, telecommunications signal, and softwaredistribution package, for example. Depending on the processing powerneeded, the computer program may be executed in a single electronicdigital processing unit or it may be distributed amongst a number ofprocessing units.

The present invention is applicable to radio telecommunication systemsdefined above but also to other suitable telecommunication systems. Theprotocols used, the specifications of mobile telecommunication systems,their network elements and subscriber terminals, develop rapidly. Suchdevelopment may require extra changes to the described embodiments.Therefore, all words and expressions should be interpreted broadly andthey are intended to illustrate, not to restrict, the embodiment. Itwill be obvious to a person skilled in the art that, as technologyadvances, the inventive concept can be implemented in various ways. Theinvention and its embodiments are not limited to the examples describedabove but may vary within the scope of the claims.

The invention claimed is:
 1. A method of providing radio communication,the method comprising: receiving a broadband signal and correlatingsub-bands of the received broadband signal to detect a first controlmessage on at least one of the sub-bands of the received broadbandsignal, the first control message being a narrowband control message,the first control message comprising a pilot sequence and an identifier,the identifier representing an identity of a transmitter of the firstcontrol message; determining the identity of the transmitter of thefirst control message from the identifier of the first control messagein response to the detection of the first control message on at leastone of the sub-bands of the received broadband signal; determining usingthe first control message a preferred sub-band of the transmitter of thefirst control message; and causing, by a receiver of the first controlmessage, transmission of a second control message on a first sub-banddetermined as the preferred sub-band, the second control messagecomprising information in addition to and different from the firstsub-band on which the second control message is transmitted thatpresents a second sub-band on which a third control message responsiveto the second control message shall be transmitted, wherein the secondsub-band is different from the first sub-band and is selectedexclusively on the basis of the second sub-band being preferred by thereceiver of the first control message regardless of whether the secondsub-band is preferred by the transmitter of the first control message,wherein the second sub-band is not indicated in the first controlmessage or preferred by the transmitter of the first control message. 2.The method of claim 1, wherein the second control message is arequest-to-send message indicating a quality-of-service classificationfor data transmission, and the third control message is a clear-to-sendmessage comprising at least a channel allocation allocating at least onesub-band for the data transmission, the channel allocation determined onthe basis of a quality-of-service classification.
 3. The method of claim2, wherein the third control message further specifies other parametersfor the data transmission.
 4. The method of claim 2, wherein therequest-to-send message comprises an information element allocating asub-band for an acknowledgment message of the data transmission, whereinthe acknowledgment message is allocated to a different sub-band that isdifferent from the sub-band allocated to the data transmission.
 5. Themethod of claim 1, wherein a pilot sequence comprised in the thirdcontrol message is shorter than the pilot sequence of the first controlmessage.
 6. The method of claim 1, further comprising: determining, as aresult of said correlation, whether the first control message wasreceived as a surface radio wave or as an ionospheric reflection wave;and utilizing a different communication procedure depending on whether acommunication link with the transmitter is the surface radio wave or theionospheric reflection wave.
 7. The method of claim 1, furthercomprising: receiving the third control message on the second sub-band,the third control message comprising a channel allocation indicating oneor more sub-bands for data transmission with the transmitter, the one ormore sub-bands comprising at least one different sub-band that isdifferent from the second sub-band; and performing data transmissionwith the transmitter on the one or more sub-bands.
 8. The method ofclaim 1, further comprising maintaining, on the basis of the receivedfirst control message and other received narrowband control messages, alist of sub-bands preferred by network nodes of a wireless network. 9.An apparatus comprising: a processing device; and a storage devicestoring instructions that, when executed by the processing device, causethe processing device to perform operations comprising: receiving abroadband signal and correlating sub-bands of the received broadbandsignal to detect a first control message on at least one of thesub-bands of the received broadband signal, the first control messagebeing a narrowband control message, the first control message comprisinga pilot sequence and an identifier, the identifier representing anidentity of a transmitter of the first control message; determining theidentity of the transmitter of the first control message from theidentifier of the first control message in response to the detection ofthe first control message on at least one of the sub-bands of thereceived broadband signal; determining using the first control message apreferred sub-band of the transmitter of the first control message; andcausing transmission of a second control message on a first sub-banddetermined as the preferred sub-band, the second control messagecomprising information in addition to and different from the firstsub-band on which the second control message is transmitted thatpresents a second sub-band on which a third control message responsiveto the second control message shall be transmitted, wherein the secondsub-band is different from the first sub-band and is selectedexclusively on the basis of the second sub-band being preferred by areceiver of the first control message regardless of whether the secondsub-band is preferred by the transmitter of the first control message,wherein the second sub-band is not indicated in the first controlmessage or preferred by the transmitter of the first control message.10. The apparatus of claim 9, wherein the second control message is arequest-to-send message indicating a quality-of-service classificationfor data transmission, and the third control message is a clear-to-sendmessage comprising at least a channel allocation allocating at least onesub-band for the data transmission, the channel allocation determined onthe basis of a quality-of-service classification.
 11. The apparatus ofclaim 10, wherein the third control message further specifies otherparameters for the data transmission.
 12. The apparatus of claim 10,wherein the request-to-send message comprises an information elementallocating a sub-band for an acknowledgment message of the datatransmission, wherein the acknowledgment message is allocated to adifferent sub-band that is different from the sub-band allocated to thedata transmission.
 13. The apparatus of claim 9, wherein a pilotsequence comprised in the third control message is shorter than thepilot sequence of the first control message.
 14. The apparatus of claim9, wherein the operations further comprise: determining, as a result ofsaid correlation, whether the first control message was received as asurface radio wave or as an ionospheric reflection wave; and utilizing adifferent communication procedure depending on whether a communicationlink with the transmitter is the surface radio wave or the ionosphericreflection wave.
 15. The apparatus of claim 9, wherein the operationsfurther comprise: receiving the third control message on the secondsub-band, the third control message comprising a channel allocationindicating one or more sub-bands for data transmission with thetransmitter, the one or more sub-bands comprising at least one differentsub-band that is different from the second sub-band; and performing datatransmission with the transmitter on the one or more sub-bands.
 16. Theapparatus of claim 9, wherein the operations further comprisemaintaining, on the basis of the received narrowband control message andother received narrowband control messages, a list of sub-bandspreferred by network nodes of a wireless network.
 17. A computer programproduct embodied on a non-transitory distribution medium readable by acomputer and comprising program instructions that, when executed by anapparatus, perform operations comprising: receiving a broadband signaland correlating sub-bands of the received broadband signal to detect afirst control message on at least one of the sub-bands of the receivedbroadband signal, the first control message being a narrowband controlmessage, the first control message comprising a pilot sequence and anidentifier, the identifier representing an identity of a transmitter ofthe first control message; determining the identity of the transmitterof the first control message from the identifier of the first controlmessage in response to the detection of the first control message on atleast one of the sub-bands of the received broadband signal; determiningusing the first control message a preferred sub-band of the transmitterof the first control message; and causing transmission of a secondcontrol message on a first sub-band determined as the preferredsub-band, the second control message comprising information in additionto and different from the first sub-band on which the second controlmessage is transmitted that presents a second sub-band on which a thirdcontrol message responsive to the second control message should betransmitted, wherein the second sub-band is different from the firstsub-band and is selected exclusively on the basis of the second sub-bandbeing preferred by a receiver of the first control message regardless ofwhether the second sub-band is preferred by the transmitter of the firstcontrol message the second sub-band is not indicated in the firstcontrol message or preferred by the transmitter of the first controlmessage.
 18. The computer program product of claim 17, wherein thesecond control message is a request-to-send message comprisingparameters of the data transmission, and the third control message is aclear-to-send message comprising a channel allocation determined on thebasis of the parameters comprised in the request-to-send message. 19.The computer program product of claim 18, wherein the request-to-sendmessage comprises an information element allocating a sub-band for anacknowledgment message of the data transmission, wherein theacknowledgment message is allocated to a different sub-band than thesub-band allocated to the data transmission.
 20. The computer programproduct of claim 17, wherein a pilot sequence comprised in the thirdcontrol message is shorter than the pilot sequence of the first controlmessage.
 21. The computer program product of claim 17, wherein theoperations further comprise: determining, as a result of saidcorrelation, whether the first control message was received as a surfaceradio wave or as an ionospheric reflection wave; and utilizing adifferent communication procedure depending on whether a communicationlink with the transmitter is the surface radio wave or the ionosphericreflection wave.
 22. The computer program product of claim 17, whereinthe operations further comprise: receiving the third control message onthe second sub-band, the third control message comprising a channelallocation indicating one or more sub-bands for data transmission withthe transmitter, the one or more sub-bands comprising at least onedifferent sub-band that is different from the second sub-band; andperforming data transmission with the transmitter on the one or moresub-bands.
 23. The computer program product of claim 17, wherein theoperations further comprise maintaining, on the basis of the receivedfirst control message and other received narrowband control messages, alist of sub-bands preferred by network nodes of a wireless network. 24.An apparatus comprising: a processing device; and a storage device tostore instructions that, when executed by the processing device, causethe processing device to perform operations comprising: causing, by theapparatus, transmission of a first control message on a first sub-band,the first control message comprising a pilot sequence and an identifier,the identifier representing an identity of the apparatus, the firstcontrol message indicating a preferred sub-band of the apparatus;receiving, by the apparatus, from a second apparatus a second controlmessage on a second sub-band representing the preferred sub-band, thesecond control message comprising information indicating a thirdsub-band on which a third control message responsive to the secondcontrol message shall be transmitted, wherein the third sub-band isdifferent from the second sub-band and is selected exclusively on thebasis of the third sub-band being preferred by the second apparatusregardless of whether the third sub-band is preferred by the apparatus;and causing, by the apparatus, transmission of the third control messageto the second apparatus on the third sub-band, wherein the secondsub-band is not indicated in the first control message or preferred bythe transmitter of the first control message.
 25. The apparatus of claim24, wherein the second control message comprises a quality-of-serviceclassification for data transmission, wherein the operations furthercomprise: determining, on the basis of the received second controlmessage and the quality-of-service classification, one or more sub-bandsand other parameters for which the data transmission shall betransmitted in accordance with; and inserting, in the third controlmessage, at least one information element indicating the determined oneor more sub-bands and further indicating the other parameters for whichthe data transmission shall be transmitted in accordance with.